Christopher has taken 4 tests and his average score so far is 85. If he gets 100, a perfect score, on the remaining 2 tests, what will his new average be?
Explanation: If he gets 100 on the remaining tests, the sum of his scores will be $4 \cdot 85 + 2 \cdot 100 = 540$ His overall average will then be $540 \div 6 = 90$.